In a new paper published in Nature Geoscience, a team of earth scientists from the Vrije Universiteit Amsterdam, University of Lisbon and Johannes Gutenberg University has investigated the seafloor and deeper earth layers in a part of the Atlantic Ocean, southwest of Portugal. This area is home to the very largest earthquake ever observed in Europe: the Great Lisbon Earthquake of 1755. This earthquake destroyed the entire city of Lisbon, killed some 50,000 people in Portugal and caused tsunami waves that reached as far north as the Netherlands and caused local damage in several cities.
Research into subsurface structure essential for predictions
''To understand the origin of this earthquake, and to be able to predict where and when another devastating earthquake will occur in the future, research into the structure of the subsurface and the geodynamic stresses in the subsurface in this area is necessary,'' explains earth scientist Wouter Schellart. ''Therefore, our research included both detailed mapping of the subsurface structure through seismic and structural investigations, as well as geodynamic computer modelling that shows how that structure came about, and what forces, stresses and plate tectonic motions are involved.''
Unique discovery under ocean floor
The seismic survey has discovered a huge and unusual drip-like structure located under the so-called Horseshoe Abyssal Plain, a flat stretch of ocean floor about five km deep southwest of Portugal. This drip structure extends to a depth of about 200 km and consists of relatively heavy rocks, located on the edge of two tectonic plates (the African and Eurasian), which have been slowly pressed down like a drop by the two converging plates. Schellart: ''Such drop structures have been discovered before in other places on Earth, but always under continents. This is the first time such a structure has been discovered under an ocean floor.'' Geodynamic computer models were used to simulate how this structure can form under the ocean floor, and showed that this requires some very specific ingredients, which happen to be present in this area. The computer models have also shown how a large fault forms above this drip structure, and in a place that matches the predicted location of the 1755 earthquake.
Monitoring of possible location to predict disastrous earthquakes
As a result, the scientists have found the possible location of the fault that was responsible for the large Lisbon earthquake, and which could therefore cause such large earthquakes in the future. Future marine geophysical and geological research will have to verify the location, size and geometry of this predicted fault structure, and, if verified, monitor it to observe activity in order to better predict when another disastrous earthquake might strike.